To use Greek letters in a text, you may try using
enhanced postscript terminal just like the case of
super- / sub-scripts.

gnuplot> set terminal postscript enhanced

The Greek letters can be displayed by {/Symbol a}. This gives
"alpha" which corresponds to "a". The relation of the Symbol and
alphabet is as follows.

ALPHABET

SYMBOL

ALPHABET

SYMBOL

alphabet

symbol

alphabet

symbol

A

Alpha

N

Nu

a

alpha

n

nu

B

Beta

O

Omicron

b

beta

o

omicron

C

Chi

P

Pi

c

chi

p

pi

D

Delta

Q

Theta

d

delta

q

theta

E

Epsilon

R

Rho

e

epsilon

r

rho

F

Phi

S

Sigma

f

phi

s

sigma

G

Gamma

T

Tau

g

gamma

t

tau

H

Eta

U

Upsilon

h

eta

u

upsilon

I

iota

W

Omega

i

iota

w

omega

K

Kappa

X

Xi

k

kappa

x

xi

L

Lambda

Y

Psi

l

lambda

y

psi

M

Mu

Z

Zeta

m

mu

z

zeta

You can also specify various postscript characters by octal codes, for
example, {/243} is a pound (L) mark, {/247} is a section mark. See
ps_guide.ps which comes with gnuplot source distribution in detail.

The next example is to draw the linear function y=Alpha x+ Gamma
and two Greek letters with those values in the figure.

The interval can be controlled by the ofset options of
set {x|y}label command.

gnuplot> set xlabel "x" 0.0,1.0

This moves the X-axis label one letter upward. When you give
a positive Y ofset, the X label gets into the graph.
While the ofset is negative, distance between the X label and the
graph becomes larger. Gnuplot tries to draw a graph as large as
possible, so the graph hight becomes larger when the X-axis label
moves upward. On the contrary the graph becomes small when
the X-axis goes downward.

Y offset = 0

Y offset = +5

Y offset = -5

To adjust the Y-axis label, use set ylabel "Y-AXIS" +n,+m
where "+n" and "+m" are the Y-axis ofset options. The following
examples are to set the X ofset "+5" and "-5". This affects the width
of the graph.